Export of internal Abseil changes

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7a9e8d95f795be037aa2dce4e44809ad0166aaec by Samuel Benzaquen <sbenza@google.com>:

Make end() iterator be nullptr.
This makes the creation of and comparison with end() smaller and faster. `find()!=end()` becomes leaner.

PiperOrigin-RevId: 304681605

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8f3024979446b391b79b1b60ada7d00a504d6aa6 by Derek Mauro <dmauro@google.com>:

Fix Bazel's distdir detection and prefer double brackets (bash recommendation)

PiperOrigin-RevId: 304615725

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f1d709cb4b2b3743d548b814dd19602fb057a5e6 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 304570545

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2bbfa5bda52057e1938a96c286ad33ff64e535e0 by Gennadiy Rozental <rogeeff@google.com>:

Implement general storage case as aligned buffer.

Aside from eliminating dynamic memory allocation for flag storage, we also saving 11 bytes per int flag, 15 bytes per double and string flag.

PiperOrigin-RevId: 304511965

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9e1aed2a95d7d060f8b906fe8c67fc3ba537b521 by Derek Mauro <dmauro@google.com>:

Use reserve to make a bad_alloc less likely in endian_test

This happened once and shouldn't have happened, so it was probably
just a flake, but might as well make this change.

PiperOrigin-RevId: 304505572

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c2faf22ba2d4d66753390e6959494214895581f0 by Gennadiy Rozental <rogeeff@google.com>:

Use anonymous bit fields to enforce separation between const and mutable bit fields.

We also move init_control field (which is now safe) to save 8 bytes per flag (based on size_tester output)

PiperOrigin-RevId: 304505215

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7ec51250a84bb03e826b3caad64431e91748186a by Krzysztof Kosiński <krzysio@google.com>:

Change the buffer size in AppendNumberUnit to constexpr.

PiperOrigin-RevId: 304492779

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a6c8db1be4f421ea7b7c02f7a01b4f48bad61883 by Gennadiy Rozental <rogeeff@google.com>:

Add test cases for two word storage.

Some additional tests were added for other storage kinds as well. These came about after I started to look into a coverage output and noticed that some cases (like reading flag values via reflection) were not covered by this test at all. It does not make sense to just add tests for two word values, so I've covered other storage kinds as well.

PiperOrigin-RevId: 304432511

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2644ecc32e1215cd6451efcb2f1054fd77e7c812 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 304254681

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4949a6b20c2bb4b9b2c811f439ccb893abc08df5 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 304250274
GitOrigin-RevId: 7a9e8d95f795be037aa2dce4e44809ad0166aaec
Change-Id: I01623de87355bec5cf87cc5932a1ca44cade9aae

1 files changed, 49 insertions, 54 deletions

diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index ae42dedcd633..2ae3dce3d13d 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -46,8 +46,8 @@ namespace flags_internal {
 // by function specific to that type with a signature matching FlagOpFn.
 
 enum class FlagOp {
+  kAlloc,
   kDelete,
-  kClone,
   kCopy,
   kCopyConstruct,
   kSizeof,
@@ -63,13 +63,13 @@ using FlagOpFn = void* (*)(FlagOp, const void*, void*, void*);
 template <typename T>
 void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3);
 
-// Deletes memory interpreting obj as flag value type pointer.
-inline void Delete(FlagOpFn op, const void* obj) {
-  op(FlagOp::kDelete, obj, nullptr, nullptr);
+// Allocate aligned memory for a flag value.
+inline void* Alloc(FlagOpFn op) {
+  return op(FlagOp::kAlloc, nullptr, nullptr, nullptr);
 }
-// Makes a copy of flag value pointed by obj.
-inline void* Clone(FlagOpFn op, const void* obj) {
-  return op(FlagOp::kClone, obj, nullptr, nullptr);
+// Deletes memory interpreting obj as flag value type pointer.
+inline void Delete(FlagOpFn op, void* obj) {
+  op(FlagOp::kDelete, nullptr, obj, nullptr);
 }
 // Copies src to dst interpreting as flag value type pointers.
 inline void Copy(FlagOpFn op, const void* src, void* dst) {
@@ -80,6 +80,12 @@ inline void Copy(FlagOpFn op, const void* src, void* dst) {
 inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
   op(FlagOp::kCopyConstruct, src, dst, nullptr);
 }
+// Makes a copy of flag value pointed by obj.
+inline void* Clone(FlagOpFn op, const void* obj) {
+  void* res = flags_internal::Alloc(op);
+  flags_internal::CopyConstruct(op, obj, res);
+  return res;
+}
 // Returns true if parsing of input text is successfull.
 inline bool Parse(FlagOpFn op, absl::string_view text, void* dst,
                   std::string* error) {
@@ -195,7 +201,7 @@ constexpr FlagHelpArg HelpArg(char) {
 
 // Signature for the function generating the initial flag value (usually
 // based on default value supplied in flag's definition)
-using FlagDfltGenFunc = void* (*)();
+using FlagDfltGenFunc = void (*)(void*);
 
 union FlagDefaultSrc {
   constexpr explicit FlagDefaultSrc(FlagDfltGenFunc gen_func_arg)
@@ -253,46 +259,36 @@ using FlagUseTwoWordsStorage =
 #endif
 
 template <typename T>
-using FlagUseHeapStorage =
+using FlagUseBufferStorage =
     std::integral_constant<bool, !FlagUseOneWordStorage<T>::value &&
                                      !FlagUseTwoWordsStorage<T>::value>;
 
 enum class FlagValueStorageKind : uint8_t {
-  kHeapAllocated = 0,
+  kAlignedBuffer = 0,
   kOneWordAtomic = 1,
   kTwoWordsAtomic = 2
 };
 
 template <typename T>
 static constexpr FlagValueStorageKind StorageKind() {
-  return FlagUseHeapStorage<T>::value
-             ? FlagValueStorageKind::kHeapAllocated
+  return FlagUseBufferStorage<T>::value
+             ? FlagValueStorageKind::kAlignedBuffer
              : FlagUseOneWordStorage<T>::value
                    ? FlagValueStorageKind::kOneWordAtomic
-                   : FlagUseTwoWordsStorage<T>::value
-                         ? FlagValueStorageKind::kTwoWordsAtomic
-                         : FlagValueStorageKind::kHeapAllocated;
+                   : FlagValueStorageKind::kTwoWordsAtomic;
 }
 
-struct FlagHeapAllocatedValue {
-  using value_type = void*;
-
-  value_type value;
-};
-
 struct FlagOneWordValue {
-  using value_type = std::atomic<int64_t>;
   constexpr FlagOneWordValue() : value(UninitializedFlagValue()) {}
 
-  value_type value;
+  std::atomic<int64_t> value;
 };
 
 struct FlagTwoWordsValue {
-  using value_type = std::atomic<AlignedTwoWords>;
   constexpr FlagTwoWordsValue()
       : value(AlignedTwoWords{UninitializedFlagValue(), 0}) {}
 
-  value_type value;
+  std::atomic<AlignedTwoWords> value;
 };
 
 template <typename T,
@@ -300,9 +296,10 @@ template <typename T,
 struct FlagValue;
 
 template <typename T>
-struct FlagValue<T, FlagValueStorageKind::kHeapAllocated>
-    : FlagHeapAllocatedValue {
+struct FlagValue<T, FlagValueStorageKind::kAlignedBuffer> {
   bool Get(T*) const { return false; }
+
+  alignas(T) char value[sizeof(T)];
 };
 
 template <typename T>
@@ -347,10 +344,8 @@ struct FlagCallback {
 // The class encapsulates the Flag's data and access to it.
 
 struct DynValueDeleter {
-  explicit DynValueDeleter(FlagOpFn op_arg = nullptr) : op(op_arg) {}
-  void operator()(void* ptr) const {
-    if (op != nullptr) flags_internal::Delete(op, ptr);
-  }
+  explicit DynValueDeleter(FlagOpFn op_arg = nullptr);
+  void operator()(void* ptr) const;
 
   FlagOpFn op;
 };
@@ -416,10 +411,10 @@ class FlagImpl final : public flags_internal::CommandLineFlag {
   // it is only used inside the three routines below, which are defined in
   // flag.cc, we can define it in that file as well.
   template <typename StorageT>
-  typename StorageT::value_type& OffsetValue() const;
-  // This is an accessor for a value stored in heap allocated storage.
-  // Returns a mutable reference to a pointer to allow vlaue mutation.
-  void*& HeapAllocatedValue() const;
+  StorageT* OffsetValue() const;
+  // This is an accessor for a value stored in an aligned buffer storage.
+  // Returns a mutable pointer to the start of a buffer.
+  void* AlignedBufferValue() const;
   // This is an accessor for a value stored as one word atomic. Returns a
   // mutable reference to an atomic value.
   std::atomic<int64_t>& OneWordValue() const;
@@ -492,17 +487,8 @@ class FlagImpl final : public flags_internal::CommandLineFlag {
   // Kind of storage this flag is using for the flag's value.
   const uint8_t value_storage_kind_ : 2;
 
-  // ------------------------------------------------------------------------
-  // The bytes containing the const bitfields must not be shared with bytes
-  // containing the mutable bitfields.
-  // ------------------------------------------------------------------------
-
-  // Unique tag for absl::call_once call to initialize this flag.
-  //
-  // The placement of this variable between the immutable and mutable bitfields
-  // is important as prevents them from occupying the same byte. If you remove
-  // this variable, make sure to maintain this property.
-  absl::once_flag init_control_;
+  uint8_t : 0;  // The bytes containing the const bitfields must not be
+                // shared with bytes containing the mutable bitfields.
 
   // Mutable flag's state (guarded by `data_guard_`).
 
@@ -514,6 +500,9 @@ class FlagImpl final : public flags_internal::CommandLineFlag {
   // Has this flag been specified on command line.
   bool on_command_line_ : 1 ABSL_GUARDED_BY(*DataGuard());
 
+  // Unique tag for absl::call_once call to initialize this flag.
+  absl::once_flag init_control_;
+
   // Mutation counter
   int64_t counter_ ABSL_GUARDED_BY(*DataGuard());
   // Optional flag's callback and absl::Mutex to guard the invocations.
@@ -600,11 +589,17 @@ class Flag {
 template <typename T>
 void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) {
   switch (op) {
-    case FlagOp::kDelete:
-      delete static_cast<const T*>(v1);
+    case FlagOp::kAlloc: {
+      std::allocator<T> alloc;
+      return std::allocator_traits<std::allocator<T>>::allocate(alloc, 1);
+    }
+    case FlagOp::kDelete: {
+      T* p = static_cast<T*>(v2);
+      p->~T();
+      std::allocator<T> alloc;
+      std::allocator_traits<std::allocator<T>>::deallocate(alloc, p, 1);
       return nullptr;
-    case FlagOp::kClone:
-      return new T(*static_cast<const T*>(v1));
+    }
     case FlagOp::kCopy:
       *static_cast<T*>(v2) = *static_cast<const T*>(v1);
       return nullptr;
@@ -675,13 +670,13 @@ class FlagRegistrar {
 struct EmptyBraces {};
 
 template <typename T>
-T* MakeFromDefaultValue(T t) {
-  return new T(std::move(t));
+void MakeFromDefaultValue(void* dst, T t) {
+  new (dst) T(std::move(t));
 }
 
 template <typename T>
-T* MakeFromDefaultValue(EmptyBraces) {
-  return new T{};
+void MakeFromDefaultValue(void* dst, EmptyBraces) {
+  new (dst) T{};
 }
 
 }  // namespace flags_internal